1. Field of the Invention
This invention relates in general to solar receivers and in particular to a novel solar receiver of high efficiency.
2. Description of the Prior Art
In a solar collector, it is possible to absorb about 99% of the available energy, however, subsequent losses severely degrade the overall collector efficiency. These losses become greater as the operating temperature increases.
When solar radiation is absorbed by a conventional flat plate absorber, the absorber surface becomes hotter than its surroundings and gives up its heat not only to the working fluid but also to the environment through convection, conduction and reradiation. A glass plate is commonly placed over the absorber to minimize the convection losses. A selective surface such as a black chrome plating is often applied to the absorber so as to reduce and minimize the reradiation losses by reducing the emissivity of the plate while maintaining the absorbitivity at a high level. Such techniques are effective up to about 180.degree. F. operating temperature. Concentrating collectors are used above this temperature to further minimize the reradiation losses by reducing the area over which reradiation occurs. This improvement is somewhat marginal but generally allows reasonable efficient operation up to roughly 220.degree. F.
Above 200.degree. to 250.degree. F. it is necessary to enclose the absorber in a high vacuum which will virtually eliminate convective and conduction losses. Conventional approaches using evacuated receivers require glass to metal seals which are expensive and unreliable. Generally, glass to metal seals are accomplished by using a material which has a coefficient of expansion halfway between that of glass and metal but such seals become very unreliable over a period of time when the metal portion has a large characteristic dimension such as the diameter of the tube which carries the liquid through the collector and when the temperature cycle is over a large range. Both of these conditions exist in solar collectors of the prior art.
It has been known in the prior art to provide two concentric tubes which are welded at one end and between which a vacuum is maintained. However, such apparatuses have several disadvantages in that the sun light must go through two separate thicknesses of glasses with a 10% loss through each thickness. Also, the inside glass tube reaches a temperature equivalent to the absorber surface which might be as high as 300.degree. F. and at this temperature the emissivity of the glass is very high which reduces and negates the advantage of applying a selective coating on the absorber surface.
It is also known to provide a receiver which comprises a pair of layers of glass separated by vacuum with a selective coating on the outside diameter of the inner tube. The sun light passes through one thickness of glass and is absorbed and the inner tube becomes hot and transfers its heat to a metal member which is in intimate contact with the inner glass tube. Generally, however, the metal does not contact the glass as well as desired and losses occur. It is very difficult to apply a selective emissivity coating to glass which will withstand hard vacuum and high temperature.